Map all 754 skills to MITRE ATT&CK v19.1

- Add validated mitre_attack frontmatter to all 754 skills (286 distinct
  techniques), verified against MITRE ATT&CK v19.1 via the official
  mitreattack-python library: 0 revoked, deprecated, or invalid IDs
- Curate precise per-skill technique IDs for forensics, malware-analysis,
  threat-intel, and red-team skills (e.g. DCSync -> T1003.006,
  Kerberoasting -> T1558.003, Pass-the-Ticket -> T1550.003)
- Reconcile v19.1 tactic restructuring: Defense Evasion split into
  Stealth (TA0005) and Defense Impairment (TA0112); revoked T1562.*
  family and T1070.001/.002 remapped to active equivalents (T1685.*)
- Normalize word-split tags across 35 skills (remove filename-derived
  stopword tags, add semantic cybersecurity tags)
- Add api-reference.md for 3 skills that were missing it
- Update README ATT&CK section with accurate v19.1 tactic distribution

skills/implementing-beyondcorp-zero-trust-access-model/SKILL.md

@@ -1,7 +1,8 @@
 ---
 name: implementing-beyondcorp-zero-trust-access-model
-description: 'Implementing Google''s BeyondCorp zero trust access model to eliminate implicit trust from the network perimeter,
-  enforce identity-aware access controls using IAP, Access Context Manager, and Chrome Enterprise Premium for VPN-less secure
+description: 'Implementing Google''s BeyondCorp zero trust access model to eliminate
+  implicit trust from the network perimeter, enforce identity-aware access controls
+  using IAP, Access Context Manager, and Chrome Enterprise Premium for VPN-less secure
   application access.
 
   '
@@ -23,6 +24,12 @@ nist_csf:
 - PR.AA-05
 - PR.IR-01
 - GV.PO-01
+mitre_attack:
+- T1078
+- T1190
+- T1059
+- T1078.004
+- T1530
 ---
 
 # Implementing BeyondCorp Zero Trust Access Model